The experiments proposed in this application utilize autoimmune vasculitis model to study both the immune properties of microvascular smooth muscle (SM) and the pathogenesis of this type of vasculitis. In the model, cultured murine SM activates syngeneic CD4+ (helper) T cells elicit vasculitis upon transfer to syngeneic hosts. It is known that SM presents exogenous antigen to a particular subset (Th1) of T cells effectively. The main hypothesis driving these studies is that SM presents endogenous (SM specific) antigen to the T cells that in turn elicit vasculitis. Four key questions will be addressed by four specific aims: 1) Are T cells activated by SM antigen? SM will be fed to antigen-presenting cells such as macrophages and dendritic cells to determine whether SM antigen can re-activate previously anergized (by SM) T cells. 2) What antigen presenting elements does SM possess that results in antigen presentation specifically to Th1 cells? The antigen-presenting machinery of SM cells will be genetically re-engineered by transfecting combinations of key MHC and co-stimulatory molecules. 3) What role does anti-SM play in the pathogenesis of vasculitis? Vasculitic mouse sera will be checked for anti-SM isotype, Western blots with the sera and SM will be done and antibody dependent cellular cytotoxicity studies will be performed in vitro. 4) What are the constituents of the vasculitic lesion? Infiltrating cells will be phenotyped and the presence of complement will be sought by immunohistochemistry. It is reasoned that these studies are important, because it is not known what properties SM has that makes it able to specifically activate, and present antigen to a specific T cell subtype. A better understanding of these properties will also add insight into the antigen-presenting properties of other "non-professional" antigen-presenting cells. This vasculitic model closely resembles human cellular vasculitic syndromes and a better understanding of the causal mechanisms of disease in this model may be directly relevant to human vasculitides. Finally, SM is underappreciated as an immune cell; it plays immuno-biological roles in vasculitides and atherosclerosis and it is a key barrier cell in post capillary venules from which leukocytes emigrate to tissues in both immune and inflammatory states.